Why you should have a specific inbound dial peer configured on an H323 IOS gateway

If no incoming dial peer is matched by the router or gateway, the inbound call leg is automatically routed to a default dial peer (POTS or Voice-Network). This default dial peer is referred to as dial-peer 0 or pid:0.

Note: There is an exception to this statement. Cisco voice and dial platforms, such as the AS53xx and AS5800, require that a configured inbound dial peer is matched for incoming POTS calls to be accepted as voice calls. If there is no inbound dial peer match, the call is treated and processed as a dial-up (modem) call.

Dial-peer 0 has a default configuration that cannot be changed. The default dial-peer 0 fails to negotiate non-default capabilities, services, and applications such as:

• Non-default Voice-Network capabilities: dtmf-relay, no vad, and so forth.
• Direct Inward Dial (DID)
• TCL Applications

Dial-peer 0 for inbound VoIP peers has this configuration:
any codec
vad enabled
no rsvp support
fax-rate voice
no dtmf relay
IP precedence 0

Note: The default DSCP for voice is EF codepoint 101110 (RFC 2598), and the default DSCP for signaling is AF31 codepoint 011010 (RFC 2597). The default dial peer does not mark packets to DSCP 0. All voice packets on the routers are marked by default (this can be overridden by the dial peer), signaling with AF31 and media with EF. Calls that match the default dial peer 0 should also have this behavior.

Dial-peer 0 for inbound POTS peers has this configuration:
no ivr application

I would imagine then that you wouldn't want a situation where you have no control of all incoming call legs, plus if there is any dtmf-relay mismatch, calls might not be routed properly or fail.It is therefore recommended that you configure an inbound dial peer for all incoming call legs.
Before I tell you how to do this,lets take a few seconds to look at how the router selects an inbound POTs dial peer based on the information elements in the setup message of the call with the dial peer attributes.
The router attempts to match these items in the following order:
1. Called number(DNIS) with the incoming called-number command
2. Calling number (ANI) with the answer-address commend
3. Calling number(ANI) with the destination-pattern command
4. Incoming voice port with configured voice port

The following configuration is recommended for incoming POTs dial peers:

dial-peer voice tag pots
incoming called-number .
port X/X/X

Note: that (.) is the only wildcard for incoming called-number. Also ensure to add the direct-inward-dial command if DID is supported.
For VOIP dial peers,the same routing matches as with the POTs dial-peer(except the direct-inward-dial cannot be used).If there are no matches with the incoming called-number,answer-address and destination-pattern (matching on the voice port is not an option for voip dial-peers),then the default dial-peer is used.It is recommended to always use a dial-peer with the incoming called-number command to ensure we always have a match.
Don't forget to change properties like CODECS,VAD(Voice Activity Detection) and DTMF-RELAY to match what you desire.